Method and apparatus for waste digestion using multiple biological processes

ABSTRACT

Methods and apparatus for treating waste streams utilize controlled introduction of microscopic gaseous bubbles to create, in a single vessel, as many as three separate biological environments in discrete, stratified zones. In preferred embodiments, bubbles of air are introduced at the bottom of the vessel, creating an aerobic zone in this vicinity. Depletion of oxygen by microorganisms resident in this layer creates an anoxic zone that drifts upward, establishing itself above the aerobic layer. The two layers remain segregated due to the intolerance of aerobic microorganisms for the overlying anoxic environment, the sharpness of the interface depending on the degree of intolerance. Preferably, the treatment vessel is used in conjunction with a clarifier in an internal recycle configuration; clear water from the top of the clarifier is conducted away from the system as treated effluent, while a portion of the biomass settling at the bottom of the clarifier is returned to the head of the treatment vessel through a hydrocyclone and screen arrangement to remove inert content, thereby substantially increasing the efficiency of biological waste-digestion processes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to biological treatment of contaminatedliquids and effluent, and more particularly to apparatus and methods forthe simultaneous application of multiple biological processes towastewater treatment, biopurification processes and desalinationpretreatment.

2. Description of the Related Art

Biological processes to treat contaminated water take many forms.Generally these involve exposure of the waste stream to one or moreforms of microorganism that stabilize or digest various of thecontaminants. The microorganisms are chosen to complement the wastestream both in terms of sewage contents and chemical environment, sinceany species of microorganism favors a particular environment withlimited tolerance for variation. For example, the activated sludgeprocess utilizes aerobic bacteria that remove the soluble biologicaloxygen demand (BOD) from wastewater. Practice of this process generallyinvolves conducting wastewater into an aeration basin containing asuspension of digestive microorganisms, thereby forming a "mixed liquor"that is aerated to furnish oxygen for respiration of biomass; thebiomass sorbs, assimilates and metabolizes the BOD of the wastewater.After a suitable period of aeration, the mixed liquor is introduced intoa clarifier in which the biomass settles, allowing the treatedwastewater to overflow into an outlet effluent stream.

An important aspect of traditional wastewater treatment is adequateagitation of the mixed liquor in order to speed contact between thedigestive microorganisms and waste materials, which may be suspended ordissolved in the wastewater. Indeed, an optimal amount of turbulence isgenerally dictated more by economics than by process requirements; highagitation rates are theoretically the most desirable, but are alsoexpensive to attain. See, e.g., U.S. Pat. Nos. 4,961,854, 4,056,465 and3,964,998.

An exception to this practice involves the use of fixed-growth media,where the biological organisms are maintained on fixed supports ratherthan dispersed in suspension. In this case mixing is avoided to preventshear that might remove the biological attached growth. The applicationof fixed-growth systems is ordinarily restricted to soluble,non-particulate contaminants; in addition, these processes are limitedin loading capacity by the surface area of the biological support andthe diffusion characteristics of the waste stream.

Fluidized-bed systems represent a combination of suspension andfixed-growth processes, but require added media for surface area, mixingsufficient to maintain homogeneity of the media and its attachedbiological growth, and periodic or continuous removal of the media forregeneration.

All of these systems ordinarily are limited to one category ofmicroorganism, since differing biological processes vary significantlyin terms of multiplication rates, optimum conditions, and preferredinputs and waste products. Most generally, microorganisms for wastewatertreatment include aerobic, anaerobic and anoxic species, all of whichare sustained by very different (and mutually inconsistent)environments. Process conditions can also restrict the applicability ofa particular biological approach. For example, the optimal biologicalprocess for a particular wastewater composition might require a longersolids retention time than that afforded by economically feasiblecomplete-mix processes, and exhibit greater throughput needs than can bemet with fixed-film and fluid-bed film reactors.

This is unfortunate, since frequently a combination of biologicalprocesses would be ideal for treatment of a particular wastecomposition. Thus, it might be advantageous to combine both nitrifyingand denitrifying agents, but the former require substantial dissolvedoxygen while the latter can only tolerate minimal (if any) dissolvedoxygen. Although some progress in combining processes has been achievedusing facultative lagoons, these constructions generally require acresof surface area, are used to process only small amounts of waste, andremain at the mercy of natural weather conditions that canuncontrollably alter process conditions and affect biological viability.

DESCRIPTION OF THE INVENTION Objects of the Invention

Accordingly, it is an object of the present invention to treatwastewater or biological sludge in a biologically optimal manner.

It is another object of the invention to facilitate simultaneous use ofmultiple biological processes.

It is a further object of the invention to allow the coexistence, in asingle vessel, of multiple biological processes having inconsistentenvironmental requirements.

Still another object of the invention is to create controlled, multiplebiological environments in a single vessel without mixing.

Yet another object of the invention is to control and maintain therelative proportions of each separate biological environment bynon-turbulent adjustment of conditions.

It is still another object of the invention to process waste in a singlevessel through simultaneous use of aerobic, anoxic and anaerobicmicroorganisms.

It is yet a further object of the invention to quiescently introduce gasinto a single-vessel, multiple-process environment to supply and/orremove nutrients and biological byproducts.

It is another object of the invention to process waste in a singlevessel with multiple biological environments whose relative ratios arecontrolled to meet target oxidation-reduction potentials at one or moreeffluent points.

Another object of the invention is to remove inert substances frominternally recycled waste streams to a treatment vessel.

Still another object of the invention is to remove from recycled wastestreams inert substances having sizes similar to those ofwaste-digestive microorganisms without depleting the stream of suchmicroorganisms.

Summary of the Invention

The foregoing objects are efficiently attained through controlledintroduction, into a single treatment vessel, of microscopic gaseousbubbles to create as many as three different biological environments indiscrete, stratified zones. In preferred embodiments, bubbles of air areintroduced at the bottom of the vessel, creating an aerobic zone in thisvicinity. Depletion of oxygen by microorganisms resident in the aerobiczone creates an anoxic region that drifts upward, establishing itselfabove the aerobic layer. The two layers remain segregated due to theintolerance of aerobic microorganisms for the overlying anoxicenvironment, with the sharpness of the interface depending on the degreeof intolerance. If the anoxic zone is populated by denitrifyingmicroorganisms, which are ideally suited to such a zone, theirproduction of gaseous or dissolved nitrogen creates an overlyinganaerobic zone substantially or fully depleted of oxygen, nitrates andnitrites; in addition, under quiescent (i.e., limited mixing)conditions, the dissolved nitrogen gas forms an insulation layer betweenanaerobic and anoxic zones, thereby contributing to segregation of thesezones. Molecular diffusion among zones is sufficient to keep all zonessupplied with nutrient and prevent accumulation of dissolved byproductsdespite the absence of mechanical mixing.

Equipment for generating bubbles suitable for use in connection with thepresent invention is described in U.S. Pat. No. 5,316,682, the entiredisclosure of which is hereby incorporated by reference. Such equipmentavoids turbulent conditions that would be fatal to practice of thepresent invention. Indeed, as noted in that patent, quiescent conditionscan promote formation of a beneficial covering layer of biologicalsolids. In the present case it has been further recognized that properlycontrolled introduction of such bubbles into waste liquids comprising acombination of microorganisms that require mutually antagonisticenvironments can result in their simultaneous accommodation; ideally,these waste-digestive microorganisms include aerobic, anoxic andanaerobic varieties. (As used herein, the term "waste-digestivemicroorganism" refers to any self-sustaining microscopic organism, suchas bacteria or protozoa, capable of digesting organic waste componentsinto mineral or gaseous products.)

Thus, in a first aspect, the invention comprises methods and apparatusfor achieving multiple discrete zones of environmentally incompatiblewaste-digestive microorganisms in a single vessel. In a second aspect,the invention comprises means for automatically controlling certaincritical parameters so as to maintain, in the treatment vessel, a targetlevel of at least one biological indicator. This indicator is selectedin accordance with the type of waste being treated. Ordinarily, theindicator will be at least one of ammonia level; soluble nitrate level;soluble nitrite level; and oxidation-reduction potential (ORP). Thelatter indicator measures, on an arbitrary scale, the electromotiveposition of the bulk waste liquid. Key controlled parameters include thegas (generally air) content of the bulk liquid and the degree ofturbulence.

Preferably, the treatment vessel is used in conjunction with a clarifierin an internal recycle configuration; clear water from the top of theclarifier is conducted away from the system as treated effluent,carrying soluble mineral residues, while most or all of the biomasssettling at the bottom of the clarifier is returned to the head of thetreatment vessel through a hydrocyclone and screen arrangement to removeinert content. It has been found, quite surprisingly, that removal ofinert content using the screen/cyclone combination substantiallyincreases the efficiency of biological waste-digestion processes(including, but by no means limited to, those utilizing themultiple-zone system of the present invention). Indeed, because much ofthe inert material removed in accordance herewith is of a sizecomparable to that of the waste-digestive microorganisms themselves, itsmere presence in the waste stream heretofore has gone largely unnoticed.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing discussion will be understood more readily from thefollowing detailed description of the invention, when taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a schematic depiction of a preferred system implementing thepresent invention; and

FIG. 2 is a schematic depiction of the control and gas micronizerapparatus of the present invention, suitable for use in conjunction withthe treatment vessel shown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

1. General System Configuration

Refer first to FIG. 1, which illustrates a suitable system, indicatedgenerally at reference numeral 10, for the treatment of waste liquid inaccordance with the present invention. Waste liquid flowing into thesystem 10 first encounters a gross filter screen 12 having a mesh sizethat may range from 25 mm down to 6 mm; the large items of trashaccumulating on the receiving face of screen 12 are periodicallyremoved, as indicated by the arrow 14. The screened liquid is conductedalong a conduit 16 to a bioreactor treatment vessel or tank 18, wherewaste-digestive organisms are allowed to digest its biodegradablecomponents. Preferably, the waste liquid contains at least two differentforms waste-digestive microorganism each requiring a different chemicalenvironment for survival or at least optimum performance. As discussedin greater detail below, the different forms of microorganism can alsobe complementary in the sense that each degrades a different type ofwaste. If the waste liquid lacks a desirable form of digestive organism,this can be introduced directly into vessel 18.

After a start-up period that depends on the concentration of digestiblewaste in the influent stream, the mixed liquor is continuously conductedfrom vessel 18 over a conduit 20 to a clarifier 22, where settling takesplace, as new influent reaches vessel 18 via conduit 16. Biologicalsolids (so-called "activated sludge") are continuously withdrawn from anoutlet point near the bottom of clarifier 22 and recycled to vessel 18via an internal recycle loop 30, while a clarified portion of the liquidis continuously withdrawn (or allowed to overflow) from an outlet pointnear the top of clarifier 22 and discarded, thereby maintaining asuitable concentration of biosolids within the system.

Recycle loop 30 comprises a first conduit 32 leading from clarifier 22to a pump 33, which conveys activated sludge to a removal subsystem 34and a second conduit 36 leading back to the head of vessel 18. Removalsubsystem 34, the details of which are described hereinbelow, isdesigned to remove inert and nondegradable materials from the biosolidsreturn stream, thereby improving the efficiency of waste treatment.

This general system configuration is suitable for use with a vessel 18configured for multi-zone waste treatment, as described immediatelybelow; however, because of the general utility of removal subsystem 34,it can also be employed with entirely conventional (i.e., single-zone)treatment processes. Similarly, the utility of multi-zone treatment isnot limted to recycle configurations that include a removal system inaccordance herewith.

2. Multi-Zone Treatment Vessel

Refer now to FIG. 2, which illustrates an apparatus that creates andfacilitates maintenance of up to three zones in vessel 18. Generally,the apparatus includes a gas micronizer loop and a feedback controlsystem that governs its operation.

The micronizer loop, indicated generally at reference numeral 50,generates microscopic bubbles and introduces them into vessel 18 in amanner that does not cause excessive turbulence. Micronizer loop 50includes a micronizer element 52 that introduces microscopic bubblesinto a stream of liquid flowing therethrough. As more fully discussed inthe '682 patent, element 52 preferably includes a cylindrical porousmembrane coupled at each end to a tapered conduit. Surrounding themembrane is a coaxial housing, sealed with respect to the membrane andcapable of containing gas under elevated pressure. Gas is provided tothe housing of element 52 through a sealed, one-way inlet. Accordingly,fluid introduced into either tapered conduit passes axially through thebore of element 52, where it acquires bubbles of gas radiallypenetrating the pores of the cylindrical membrane.

Waste fluid is continuously withdrawn from vessel 18 through a valve 54by means of a motor-driven pump 56 and provided to the inlet ofmicronizer 52. A source of gas (preferably air) 58 feeds micronizer 52through a valve 60 to form bubbles in the liquid passing therethrough.Upon exiting from micronizer 52, the aerated liquid is reintroduced intothe bottom of vessel 18. Introduction of the aerated liquid, whichcontains submicron bubbles and transports them throughout the bottomregion of vessel 18, occurs without substantial turbulence. This isensured by employing bubbles having mean diameters less than one micron,stored potential energies of at least 10 lbm/ft² -sec² (where lbm ispounds mass), or a work/area factor of at least 0.5 lbf/ft (where lbf ispounds force). Preferably, stored potential energy exceeds 100 lbm/ft²-sec² and the work/area factor exceeds 3 lbf/ft.

So long as aerobic and non-aerobic (i.e., anoxic and/or anaerobic)microorganisms exist in the mixed liquor, two or more distinct,stratified chemical environments will develop in vessel 18.Representative aerobic genera, present in a wide variety of sludgecompositions, include the bacteria Acinetobacter, Pseudomonas, Zoogloea,Achromobacter, Flavobacterium, Norcardia, Bdellovibrio, Mycobacterium,Sphaerotilus, Baggiatoa, Thiothrix, Lecicothrix and Geotrichum, thenitrifying bacteria Nitrosomonas and Nitrobacter, and the protozoaCiliata, Vorticella, Opercularia and Epistylis; anoxic genera alsotypically present include the denitrifying bacteria Achromobacter,Aerobacter, Alcaligenes, Bacillus, Brevibacterium, Flavobacterium,Lactobacillus, Micrococcus, Proteus, Pseudomonas and Spirillum; andanaerobic organisms typically present include Clostridium spp.,Peptococcus anaerobus, Bifidobacterium spp., Desulfovibrio spp.,Corynebacterium spp., Lactobacillus, Actinomyces, Staphylococcus andEscherichia coli. Aerobic nitrifiers oxidize ammonia or amine compounds(such as amino acids) to nitrite and finally to nitrate, while anoxicdenitrifiers reduce nitrate to nitrate and finally to nitrogen gas. Thesimultaneous presence of nitrifiers and denitrifiers has been foundhighly useful in reducing large quantities of soluble carbonaceous BOD,as well as nitrogen-containing organics, into gaseous products. It isbelieved that soluble nitrite crosses the interface between aerobic andanoxic zones in large quantities before its conversion, by thenitrifiers, into nitrate; in the anoxic zone, denitrifiers convert thenitrite into nitrogen gas, resulting in an overall net conversion ofchemically bound nitrogen into nitrogen gas (which helps maintainseparation between anoxic and anaerobic zones in a quiescentenvironment).

Assuming the presence of all three types of microorgansism, threeenvironments--shown as Zones I, II and III in FIG. 2--develop undersufficiently quiescent conditions. As noted previously, the absence ofone or more forms of microorganism can be rectified, if desired, bytheir direct introduction into vessel 18. Indeed, merely pouring severalgallons of activated sludge into the vessel will ordinarily furnish asufficient seed population of all three classes of organism to generatethe zones after a suitable growth period.

Although the foregoing system is entirely adequate to effect multi-zonewaste treatment, it is desirable to add some degree of control to attaintarget levels of indicators important to the treatment of particularwaste compositions. The important indicators, as noted previously,include ammonia level; soluble nitrate level; soluble nitrite level; andORP. These indicators can generally be brought within limits appropriateto the particular type of waste composition by adjusting processparameters such as the air content of the bulk liquid and the degree ofturbulence imparted thereto. Desirably, the turbulence imparted to thecontents of vessel 18 by the delivered air does not exceed a meanvelocity gradient of 100 sec⁻¹ ; 40 sec⁻¹ is a typical working value,and 10 sec⁻¹ is ideal. However, below this level, changes in mixingenergy can be used to control process conditions.

For present purposes, the mean velocity gradient G is given by

    G=(P/μV).sup.1/2

where P is the power requirement or mixing horsepower from aeration inft-lb/sec, μ is the dynamic viscosity viscosity in lb-sec/ft² and V isthe tank volume in ft³. P is given by

    P=p.sub.a V.sub.a ln(p.sub.c /p.sub.a)

where p_(a) is atmospheric pressure in lb/ft², V_(a) is the volume ofintroduced air at atmospheric pressure in ft³ /sec, and p_(c) is thepressure, in lb/ft², at the point of air discharge into the fluid, or by

    P=35.28Q.sub.a ln((h+33.9)/33/9)

where Q_(a) is the air flow, in ft³ /min, into the fluid at atmosphericpressure, and h is the air pressure at the point of discharge in feet ofwater.

For example, excess free ammonia, which is ecologically harmful ifpresent in discharged effluent, in the absence of nitrates reflectsinsufficient aeration. Conversely, excess free nitrate, which can leadto groundwater contamination by solubilizing heavy metals, in theabsence of ammonia reflects excessive aeration. Excess ammonia andnitrate reflect incomplete waste mineralization and promote unwantedbiological activity at the effluent site; these indicate an insufficientdenitrifier population or excessive turbulence (the latter conditionbeing confirmed by a narrow diversity of ORP, which itself indicatesexcessive turbulence). ORP affects the health of various organismpopulations, and must therefore be kept within acceptable values. Thiscan be achieved by control of the gross average vessel oxygen contentacross all zones.

The presence of any of the foregoing adverse conditions can be detectedmanually, using appropriate chemical and/or electrolytic sensingequipment, and manual steps taken to adjust the appropriate parameter.In particular, the size of the bubbles can be controlled, within limits(as discussed in the '682 patent), by the amount of air from source 58introduced into micronizer 52 and/or by the velocity of the liquidpumped through micronizer 52. Decreasing the mean diameter of thebubbles results in their production in greater quantity, increasing thedegree of aeration. Elevating the mean diameter decreases aeration but,because the bubbles are larger, increases agitation. For most processes,control of bubble size allows the operator to exert sufficientindependent control over both aeration and agitation parameters. It isof course possible to impart additional agitation by mechanical means.

Control over process conditions can also be accomplished by automatedmeans, as illustrated in FIG. 2. A controller 62 accepts input data fromat least one sensor 64, which produces an output signal representing themagnitude of at least one of the indicators discussed above. The outputsignal may be digital or analog, depending on the characteristics ofcontroller 62. Suitable sensors are well-characterized in the art; forexample, electrode arrangements for measurement of ORP and ammonia arewidely available, as are in-line measurement devices for nitrates.Various arrangements and combinations of sensors 64 are possible; forexample, vessel 18 may be equipped with a cluster of sensors capable ofsensing all relevant indicators, or with multiple clusters spaced apartvertically in regions likely to correspond to discrete zones.

Controller 62 interprets signals from sensors 64 and, based thereon,controls valves 54 and 60 (which are, in this embodiment, electronicallyactuable) and the speed of pump 56. In addition, to facilitate evengreater control over imparted turbulence, the illustrated embodimentincludes a paddle stirrer assembly 66, the operation of which is alsogoverned by controller 62; it should be recognized, however, thatstirrer 66 is ordinarily not necessary.

Controller 62 can be an analog (e.g., voltage-controlled) device, but ispreferably a digital computer programmed with appropriate software tocarry out the analysis and control functions. In this embodiment,signals from sensors 64 are converted to digital form byanalog-to-digital converters, while the digital control signalsgenerated by controller 62 are transformed by digital-to-analogconverters into signals capable of opening and closing valves 54 and 60to a stepped or continuously selectable degree. The programmingnecessary to effectuate the analysis and control functions describedhereinabove is well within the purview of those skilled in the art, andcan readily be accomplished without undue experimentation.

3. Removal Subsystem

Removal of inert, solid substances from sludge prior to itsreintroduction into vessel 18 has been found to substantially increasethe efficiency of waste digestion. This is due, it is believed, both tobiological concentration effects (since removal of inert solids resultsin reintroduction of sludge having higher microorganism levels) and toreduction of biological toxicity (that results, e.g., from heavy metalssusceptible to removal in accordance herewith). Indeed, removal ofinerts is beneficial in virtually any biological process employing arecycle sidestream, and this aspect of the invention is therefore usefulin a wide variety of waste-treatment applications (e.g., conventionalsingle-zone tanks used independently or in series, or the multi-zonearrangement discussed above).

To understand operation of this aspect of the invention, it is importantto appreciate the variety of solids present in typical wastewater. Small(i.e., 1-250 μm in diameter) organic materials include thewaste-digestive organisms critical to waste treatment. Larger (>250 μm)organic materials represent various forms of trash. Small and largeinorganic particles include inert materials such as sand. Of thesecategories of solids, only small organics are desirably reintroducedinto vessel 18.

Conventionally, a screen having a mesh size of over 5000 μm (0.2 inch)has previously been employed in the internal recycle loop to filter thecoarsest particles from the recycle stream. Although such large meshsizes obviously discourage plugging, these screens are capable ofremoving only the very largest particles, which typically constituteonly a small fraction (generally <5% by weight) of the stream;accordingly, large amounts of solids remain to poison or at least crowdthe biology.

The removal subsystem of the present invention is a two-stage assemblythat removes, in a first stage, materials of sizes similar to those ofbiological solids (including waste-digestive microorganisms) but havingdifferent specific gravities; and in a second stage, solids ranging insize from large objects (such as those removed by conventional screens)to much smaller particles on the order to 250-350 μm. It must beemphasized that, owing to the continuous nature of the recycling loop,the order in which withdrawn sludge encounters the two stages is notcritical.

Preferably, the first stage comprises one or more hydrocyclone unitsconnected in parallel, indicated collectively at reference numeral 80(and referred to in the singlar for convenience of presentation).Hydrocyclone 80 is configured to remove small inorganic solids similarin size to biological solids but having different (and usually muchhigher) specific gravities. In particular, hydrocyclone 80 shouldprimarily remove particles in the size range 1-250 μm having specificgravities greater than 1.5.

Hydrocyclones typically operate over a range of particles size/specificgravity combinations, but exhibiting a peak efficiency dictated by theunit's size and construction. For purposes of the present invention,maximum efficiency ideally occurs at particle sizes of 50-60 μm and aspecific gravity of about 2.6. In this way, the device will capture atleast some very high density particles but avoid entrapment of desirablebiological solids, which have specific gravities of about 1.02 to 1.05.Particles collected by hydrocyclone 80 are conveyed for disposal by anoutlet line 82. The second stage comprises a static screen having a meshsize between 50 and 500 μm, and preferably 250 μm. Because virtually allwaste-degradative biological material is usually no larger than 200 μm,these pass through screen 86 and are reintroduced into vessel 18.Screenings are conveyed for disposal along an outlet path 88.Notwithstanding traditional concerns over possible clogging of screenshaving such small mesh sizes, this problem has been found not to occur.It is likely that most of the sludge-borne solids are much larger thanthan the screen mesh, and simply rest against the screen withoutclogging the pores; in addition, accumulation of large particles canalso act to restrain smaller particles that might otherwise causeclogging problems.

The screen/hydrocyclone removal arrangement 34 not only removesotherwise troublesome inert solids, but also facilitates independentcontrol of the ratio of inert content to biological content. The abilityto influence this ratio (by varying the mesh size of screen 86 and theretention characteristics of hydrocyclone 80) affords the operatorgreater control over the settling characteristics of the mixed liquor.

In a representative implementation, the invention was installed in a 0.5million gallon/day (mgd) wastewater treatment plant usingactivated-sludge treatment and aerobic digestion with an influentloading of 834 lbs/day of 5-day biological oxygen demand (BOD₅ =200) and1250 lbs/day of total suspended solids (TSS). The return activatedsludge (RAS) flow along internal recycle loop 30 was maintained atapproximately 75% of the influent rate, or 260 gallons/minute (gpm).Removal subsystem 34 consisted of two 6-inch diameter, 10° cyclonesoperating in parallel at a pressure drop of 15 psig, and a static screenhaving a mesh size of 254 μm to which the overflow of the cyclones wasconducted. The screen outflow was returned to the head of the aerationbasin 18 via conduit 36, and screenings accumulated over path 88 wereallowed to fall into a screw conveyor inclined at 5° above horizontal.The screw conveyor was sprayed with approximately 0.5 gpm of recycledplant effluent to remove residual biosolids and its contents conveyed,while draining, to a dumpster for disposal. The underflow of thecyclones was discharged through conduit 82 to a secondary cyclone washerfor concentration and discharge to the same dumpster. The secondarycyclone washer consisted of a 10-gallon reservoir recirculated through a2-inch diameter, 10° cyclone at a pressure drop of 20 psig and a flowrate of 25-30 gpm. Excess liquid was returned to the head of theaeration basin 18 over conduit 36 and a concentrated solids stream of60-80% dry solids discharged for disposal.

The vessel 18 was equipped with two micronizers having the collectivecapacity to deliver 20 cubic feet per minute of air (or other suitablegas) at standard conditions of temperature and pressure (scfm) into arecirculating liquid stream flowing at a rate of 1200 gpm. Themicronizers were arranged as two separate flow loops 50. Normaloperation was found to require up to 10 scfm of air into 600 gpm ofrecirculating flow in order to achieve sufficient digestion ofbiodegradable materials in the recirculated sludge.

Excess biological solids were transferred from the outfall of the screenout of the normal process flow at a daily rate of 5000-7000 gpd at aconcentration of 0.5-1.0% to a digester having a minimum working volumeof 70,000 gallons. The overall flow into vessel 18 was chosen tomaintain it at liquid capacity during operation.

It was found that implementation of the invention produced substantialprocess benefits as compared with conventional operation:

    ______________________________________                                                        Before       After                                            ______________________________________                                        Effluent:                                                                            BOD.sub.5    5              1                                                 TSS          10             3                                          Mixed liquor suspended                                                                        5000    ppm      3100  ppm                                    solids                                                                        Coarse bubble blower                                                                          100     hp       40    hp                                     requirements, mixing and                                                      aeration                                                                      Digester aeration require-                                                                    30      hp       23    hp                                     ments                                                                         Mean cell residence time                                                                      45      day      45    day                                    Inerts residence time                                                                         45      day      1.3   day                                    Biosolids inventory                                                                           10,425  lbs      11,327                                                                              lbs                                    Inerts inventory                                                                              10,425  lbs      800   lbs                                    Clarifier loading                                                                             10.5    lbs/     6.1   lbs/                                                           day-ft.sup.2   day-ft.sup.2                           Solids removed from                                                                           834     lbs/day  600   lbs/day                                site to maintain                                                              steady state                                                                  Horsepower per mgd                                                                            270     hp       136.5 hp                                     ______________________________________                                    

It should be noted that, prior to introduction of the invention, solidsremoved from the site to maintain steady-state operation were present ata concentration of 1.6%, resulting in a net transportation burden of50,050 lbs/day. By contrast, because the present invention concentratessolids to 60% levels or better, the maximum transportation burden was600 lbs/day.

It will therefore be seen that we have developed a highly efficient andefficacious system for wastewater treatment, both in terms of expandingthe variety of available digestion processes and improving the overalllevel of digestion through removal of inert materials from the processstream. The terms and expressions employed herein are used as terms ofdescription and not of limitation, and there is no intention, in the useof such terms and expressions, of excluding any equivalents of thefeatures shown and described or portions thereof, but it is recognizedthat various modifications are possible within the scope of theinvention claimed.

What is claimed is:
 1. A method of treating liquid-borne wastecontaining at least two different types of waste-digestivemicroorganism, the different types of microorganism requiring mutuallyincompatible chemical environments and comprising an aerobic and anon-aerobic species, the method comprising:a. introducing the waste intoa treatment vessel having a bottom region; b. continuously introducing asupply of micronized air bubbles into the bottom region of thewaste-containing treatment vessel, the continuous supply of air bubblescorresponding to a quantity of introduced air thereby creating in thebottom region an aerobic zone compatible with and containing the aerobicspecies; c. controlling the quantity of introduced air and the degree ofagitation of the waste so as to produce in the waste at least onedistinct non-aerobic zone vertically adjacent to the aerobic zone havinga chemical environment compatible with and containing the non-aerobicspecies, thereby effecting biodegradation of the waste.
 2. The method ofclaim 1 wherein the bubbles possess at least one of (a) stored potentialenergies of at least 10 lbm/ft² -sec² ; and (b) work/area factors of atleast 0.5 lbf/ft.
 3. The method of claim 2 wherein the bubbles possessstored potential energies of at least 100 lbm/ft² -sec².
 4. The methodof claim 2 wherein the bubbles possess stored potential energies of atleast 500 lbm/ft² -sec².
 5. The method of claim 2 wherein the bubblespossess work/area factors of at least 3 lbf/ft.
 6. The method of claim 2wherein the bubbles possess work/area factors of at least 4 lbf/ft. 7.The method of claim 1 wherein the waste comprises aerobic, anoxic andanaerobic waste-digestive microorganisms and the quantity of introducedair and agitation are controlled so as to produce in the waste aerobic,anoxic and anaerobic zones.
 8. The method of claim 7 wherein the anoxiczone includes a population of denitrifying microorganisms.
 9. The methodof claim 8 wherein the aerobic zone includes a population of nitrifyingmicroorganisms.
 10. The method of claim 1 further comprising the step ofcontrolling the quantity of introduced air and effecting controlledagitation of the waste so as to maintain at least one biologicalindicator at a predetermined level.
 11. The method of claim 10 whereinthe indicator comprises at least one of ammonia level; soluble nitratelevel; soluble nitrite level; and oxidation-reduction potential.
 12. Themethod of claim 1 wherein the controlled agitation does not exceed amean velocity gradient of 100 sec⁻¹.
 13. The method of claim 1 whereinthe bubbles have an average diameter, and agitation is controlled byadjusting the average diameter.
 14. The method of claim 1 wherein thebubbles have an average diameter, and the quantity of introduced air iscontrolled by adjusting the average diameter.
 15. Apparatus of treatingliquid-borne waste containing at least two different types ofwaste-digestive microorganism, the different types of microorganismrequiring mutually incompatible chemical environments and comprising anaerobic and a non-aerobic species, the apparatus comprising:a. atreatment vessel having a bottom; b. means for introducing a supply ofmicronized air bubbles into the bottom of the waste-containing treatmentvessel, the continuous supply of air bubbles corresponding to a quantityof introduced air thereby creating in the bottom region an aerobic zonecompatible with and containing the aerobic species; c. means forimparting a degree of agitation of the waste; and d. control means,coupled to the bubble-introduction means, for controlling (i) thequantity of introduced air and (ii) the degree of agitation to producein the waste at least one distinct non-aerobic zone vertically adjacentto the aerobic zone, the non-aerobic zone having a chemical environmentcompatible with and containing the non-aerobic species.
 16. Theapparatus of claim 15 wherein the means for introducing a supply ofmicronized air bubbles into the bottom of the waste-containing treatmentvessel comprises:a. microporous element having an average pore size; b.means for exposing a surface of the microporous element to gas at apredetermined pressure; and c. means for moving a fluid and themicroporous element relative to one another with sufficient motive forceto achieve a predetermined fluid velocity in excess of 10 ft/sec,whereinthe average pore size of the microporous element is less than 1 micron.17. The apparatus of claim 15 further comprising means for sensing atleast one biological indicator in the vessel and generating a signalindicative thereof, the control means being configured to receive thesignal.
 18. The apparatus of claim 17 wherein the controller responds tothe signal by controlling the quantity of introduced air and effectingcontrolled agitation of the waste so as to maintain the at least onebiological indicator at a predetermined level.
 19. The apparatus ofclaim 17 wherein the sensing means senses at least one of ammonia level;soluble nitrate level; soluble nitrite level; and oxidation-reductionpotential.
 20. The apparatus of claim 15 wherein agitation is suppliedby the bubbles, the agitation means being the bubble-introduction means.21. The apparatus of claim 20 wherein the bubbles have an averagediameter, and agitation is controlled by adjusting the average diameter.22. The apparatus of claim 15 wherein the agitation means comprisesmechanical means for imparting turbulence.
 23. The apparatus of claim 15wherein the degree of agitation does not exceed a mean velocity gradientof 100 sec⁻¹.
 24. The apparatus of claim 15 wherein thebubble-introduction means produces bubbles having an average diametercontrollable by the control means, which controls the quantity ofintroduced air by adjusting the average diameter.